Effect of larger diameter rims on a bike.

koolio · Spanner Monkey Joined: 08 Apr 2011 Karma :

I've always loved large spoked rims on bikes, especially on vintage and classic bikes, love old British racers like 40s-50s Norton Internationals that looked like they had big rims.

Was wondering if anyone would know what the effect would be of adding 20" rims on both the front and rear hubs.

The stock are 19" on the front and 18" on the back of a 70s 500cc twin Honda.

Anyone have suggestions or any ideas how this would affect the handling and traction, what difficulties there are in doing such a mod (main concern is the rear wheel) and whether or not its worth doing?

Would it require a new smaller front end for example and how does this fair for the back swing arm etc.

Any pointers as always appreciated.

Please note: this is not a Bobber/Chopper project rather a cafe racer style with big rims.

stinkwheel · Bovine Proctologist Joined: 12 Jul 2004 Karma :

You'll be gearing it significantly higher. Might need to drop a tooth or two on the front sprocket to pull away cleanly.

If you go from 19-18 to 20-20 you'll be effectively dropping the front by an inch. It'll be a bit twitchier and will drop into corners more agressively.

It will be a bitch finding decent tyres.

Your speedo will under-read by a fair amount.

Practicality-wise. You might find tyres foul the swingarm, touch the rear mudguard under compression or rub on the front mudguard bracket.

I'd check the clearance and tyre availability before blowing your wad on tasty rims.
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el_oso · World Chat Champion Joined: 17 May 2008 Karma :

larger wheels have a greater rotational mass and don't react to bumps in the road as bad as small wheels.
also, as stinkwheel said higher gear ratio
as well as dropping the front you will change the center of mass. higher centre of mass would add to the quickness of steering
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Silentwalker · Two Stroke Sniffer Joined: 14 Sep 2011 Karma :

G · The Voice of Reason Joined: 02 Feb 2002 Karma :

I think a big issue may be limited tyre availability?
You'll have more options at 21", but most will be front tyres.
Overall, it seems quite likely you will make your bike handle a bit worse, but probably not massively so.

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

I would have though it would be more stable not more twitchy. But stable in a bad way, as in not wanting to lean over at all, the bike always fighting to be upright.
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stinkwheel · Bovine Proctologist Joined: 12 Jul 2004 Karma :

chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

G · The Voice of Reason Joined: 02 Feb 2002 Karma :

But bigger wheels should make it less twitchy, so possible it will balance out Smile

.
Or you may be able to get a lower profile tyre, depending on the current one on (though less likely at these sizes.)
Or may be able to adjust the ride attitude in other ways too.

weasley · World Chat Champion Joined: 16 Oct 2010 Karma :

Well strictly speaking you are raising the rear by 1" and the front by 0.5" (since 20" is the diameter, not the radius, and it is the radius which sits between the axle and the ground).
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chris-red · Have you considered a TDM? Joined: 21 Sep 2005 Karma :

koolio · Spanner Monkey Joined: 08 Apr 2011 Karma :

I'm still a little unclear on what the effects would be.

I think I've decided to stick to the stock, recently discovering that the Dunlops K82, K70 and K81 I wanted to put on do not come in a size 20.

Thanks all anyhow.

Ariel Badger · Super Spammer Joined: 02 Dec 2006 Karma :

They will make tight cornering hard at speed due to the large rotational mass and gyroscopic forces. my hillclimber has a 21 inch front rim and it is costing me time on the hills.
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Fizzer Thou · World Chat Champion Joined: 06 Aug 2011 Karma :

On the front of one of my CB550 Hondas I had this setup

https://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/Earlydays034.jpg

What I had wanted to do was to fit an 18" alloy rim to the front hub with CB750F1 discs and caliper brackets.But with the two 500/550 discs it was more than adequate stopping,with one disc having a set of Ferodo pads,with the other having Vesrah pads.This way it was twin discs in the dry,and single disc operation in the wet,as Vesrah were known to be rubbish in the wet in those days.

But on my CB750F2 I fitted a much bigger rear wheel,a 4.75x18 rim,with the front being a 3.25x18.This allowed me to fit a 160 on the back CMA wheel and a 110/80x18 on the front CMA.Both wheels were custom made for me,thus allowing what were modern tyres in their day.

https://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/My750042.jpg
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G · The Voice of Reason Joined: 02 Feb 2002 Karma :

I've done fine on various trail bikes with a 21" front and chunky 140 18" rear... Would like to try the 690 on track with ok road biased tyres.

With road (and most track, in the end) riding it does end bring a case of the rider bring the limiting factor, possibly with the question of whether to say push a bike up 30% or 50% of the bike's abilities.